Method for controlling pretilt angle direction in a liquid crystal cell

ABSTRACT

A method is disclosed for controlling a pretilt angle direction for a liquid crystal cell comprising the steps of first setting the magnitude of pretilt angle and a plurality of pretilt angle directions in an alignment layer. This first step is achieved by irradiating linearly the alignment layer with polarized or unpolarized UV light. One of the plurality of pretilt angle directions is then selected by exposing the alignment layer to UV light a second time.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a liquid crystal cell, andparticularly to a method for controlling a pretilt angle direction inliquid crystal cell in which photoreaction occur in a polymerized layerby ultraviolet irradiation.

[0002] Liquid crystals are the liquids consisting of anisotropicmolecules. The average direction of long axes of these molecules isreferred as the director of LC. The director distribution in a LC bulkis determined by its anchoring on the rigid substrates and characterizedby the direction of the axes of easy orientation, corresponding to theminimum of the surface energy of an LC, pretilt angle between the axisof easy orientation and the substrate plane, and tilt angle between thedirector of LC and the substrate plane.

[0003] In order to obtain the uniform brightness and high contrast ratioof a liquid crystal display, the LC molecules must be appropriatelyaligned after being injected between the substrates of the cell. Notonly the value of the director tilt but the direction of this tilt (i.e.direction of the axis of easy orientation) is important for normaloperation of LC devices constituting double- and multi-domain structure.Such alignment is achieved by providing an alignment layer on thesurface of the substrate. A rubbing process can be used for aligningliquid crystal molecules. In this rubbing process, a polyamide alignmentlayer is first coated on the substrate and the rubbing is performedmechanically, so that microgrooves are formed on the surface of thealignment layer. The liquid crystal molecules are thus uniformly aligneddue to the intermolecular interaction between the polyamide moleculesand the liquid crystal molecules.

[0004] In the above described rubbing process, however, defects areformed in the microgrooves which cause light scattering and random phasedistortion. Also during the rubbing process, dust and electrostaticcharges are produced in the alignment layer, so that the substrate isdamaged and yield is decreased.

[0005] To solve the aforementioned problem, photo-alignment process hasbeen recently introduced. As an example of the photo-alignment method, amethod has proposed by KOBAYASHI, etc. (SID 95 DIGEST, p. 877) in whichthe pretilt angle direction is determined by irradiating the UV lighttwice into an alignment layer consisting of polyvinylcinnamate(PVCN)based polymer, as shown in FIGS. 1A and 1B.

[0006] In particular, as shown in FIG. 1A, when the linearly polarizedUV light irradiates to alignment layer in the direction perpendicular tothe surface of the substrate 16, the alignment layer 15 becomes aphoto-polymerized due to cross linking between polymer molecules. Thebonding direction of the photo-polymer molecules depends on thepolarization direction of the linearly polarized UV light. The liquidcrystal is thus aligned according to the bonding direction of thephoto-polymer molecules.

[0007] Then, the linearly polarized UV light whose polarizationdirection is perpendicular to the polarization direction of the first UVlight is irradiated at an angle ø to the alignment layer 15. The pretiltangle of the alignment layer 15 is formed in this step and the magnitudeof the pretilt angle varies according to the irradiation angle of the UVlight. For example, the pretilt angles are approximately 0.15°, 0.26°,or 0.30°, when the irradiation angles are 30°, 40°, or 60°,respectively.

[0008] In KOBAYASHI, however, the method has some drawbacks thethermostability of the tilt angle on the PVCN based materials is poor,the scope of the pretilt angle is small and does not cover the rangeneeded for an applications, only polarized exciting light could be used.Moreover, the method requires rather complicated geometry of theirradiation and suitable for the only materials revealing thelight-induced easy axis direction perpendicular to the polarization ofthe exciting light.

SUMMARY OF THE INVENTION

[0009] It is an object of the present invention to provide a method forcontrolling pretilt angle direction for liquid crystal cell in whichcontrol of the pretilt angle direction is easy and large pretilt anglecan be obtained.

[0010] In order to achieve the object, the method for controllingpretilt angle for liquid crystal cell comprises the first step ofirradiating UV light to an alignment layer to form pretilt angle andorient the alignment axis and second step of irradiating UV light to thealignment layer in the direction of oblique to the surface. The order ofthese step is reversible.

[0011] The first and second exposures to UV light can be performed at anangle, preferably 0-60°, or perpendicular to the substrate surface.Moreover, both polarized and unpolarized UV light can be used.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIGS. 1A and 1B are views illustrating the conventional method forcontrolling pretilt angle for liquid crystal cell;

[0013]FIG. 2 is a view illustrating an UV light irradiation andbirefringence measurement apparatus;

[0014]FIGS. 3A and 3B are views illustrating method for controllingpretilt angle direction for liquid crystal cell according to firstembodiment of the present invention;

[0015]FIG. 4 is a graph showing pretilt angle dependent on a absorptionenergy of the UV light;

[0016]FIGS. 5A and 5B are views illustrating the method for controllingpretilt angle direction for liquid crystal cell according to secondembodiment of the present invention;

[0017]FIGS. 6A and 6B are views showing the method for controllingpretilt angle direction according to third embodiment of the presentinvention;

[0018]FIGS. 7A and 7B are views showing the method for controllingpretilt angle direction according to fourth embodiment of the presentinvention;

DETAILED DESCRIPTION OF THE INVENTION

[0019]FIG. 2 is a view illustrating ultraviolet(UV) irradiating andbirefringence measuring apparatus for photo-alignment process. In thisdevice, UV light generated from Hg lamp 11 is linearly polarized througha lens 12 a and a polarizer 17 c, and is directed to the alignment layer15 coated on substrate 16. A laser beam generated from a laser 18 ispulsed by beam by a chopped 23, and then it is polarized by a polarized17 a. The pulsed beam is then transmitted to the alignment layer 15through a compensator 25, through a polarizer 17 b and a lens 12 b, andfinally inputted to a digital oscilloscope 20 so that the birefringencecaused by anisotropy of the alignment layer 15 can be measured.

[0020]FIGS. 3A and 3B illustrate first embodiment of the presentinvention. In this embodiment, the alignment layer includes polysiloxanebased material or polyvinylfluorocinnamate (PVCN-F). In this alignmentlayer, pretilt angle and the direction of the easy axis are determinedby one time irradiation of the UV light. The structural formulas of thepolysiloxanecinnamate and PVCN-F are indicated below. Further, thesematerials can be used in other embodiments of the present invention.

[0021] PVCN-F:

[0022] examples of polysiloxanecinnamate include:

[0023] Z=OH, CH₃, or a mixture of OH and CH₃

[0024] M=10-100

[0025] l=1,3,4,5,6,7,8,9, or 10

[0026] K=0,1, or 2

[0027] X,X₁,Y=H,F,Cl,CN,CF₃,OCF₃,C_(n)H_(2n+1)or OC_(n)H_(2n+1)(n=1-10)

[0028] The polymer solution, the polysiloxanecinnamate or PVCN-Fsolution, is prepared using 1:1 mixture of 1,2-dichloroetane andchlorobenzene. A concentration of the solution is 20 g/l. A droplet ofthis solution is dropped in the center of the substrate and then coatedon the whole area of the substrate by a spin-coating for 20 second at2,000 rpm. As a result, a polymer film is deposited on the substrate.The thickness of the film, measured by Linnik interferometer, is 1,000Å, and can be controlled by changing the concentration of the polymersolution and/or revolution speed of a spin-coating machine used forspin-coating.

[0029] As shown in FIG. 3A, when the linearly polarized UV light 190 isirradiated to the alignment layer 15 in the perpendicular direction ofthe surface of the substrate, the direction of alignment-axis isdetermined to be perpendicular to the polarization direction of the UVlight. Further, two symmetric and bidirectional pretilt angles ø₁ arecreated on both sides of the alignment-axis. The size of the pretiltangle ø₁ is variable and depends on the duration of UV exposure, i.e.,the amount of UV energy absorbed by alignment layer 15.

[0030]FIG. 4 is a graph showing the relation between the pretilt angleand the irradiation time of the UV light, i.e., the amount of absorptionenergy of the UV light. As shown in the figure, the larger theabsorption energy is, the smaller the pretilt angle becomes.Accordingly, the direction of the alignment axis is determined by thepolarization direction of the UV light irradiation, and the size ofpretilt angle is dependent upon the amount of UV absorption energy.

[0031] After irradiating linearly polarized UV light, another light isirradiated to the layer to the direction oblique to the surface of thealignment layer, that is, one direction of two symmetric pretilt anglesis then selected by irradiating alignment layer 15 at an angle θ₁,0°<θ₁≦60° with non-linearly polarized UV light, specially unpolarized UVlight 200, such that all the molecules of alignment layer 15 areoriented at one of the symmetric pretilt angle directions, as in FIG.3B. The alignment layer 15 preferably includes polysiloxane basedmaterial or PVCN-F. In this case, the irradiation direction of thesecond UV light forms acute angle with one of two symmetric pretiltangle directions.

[0032]FIGS. 5A and 5B illustrate a second embodiment of the presentinvention. In the figure, unpolarized UV light(wavelength λ=365 nm) 210irradiates alignment layer 15 coated on the substrate 16 at an angle θ₂to the normal direction of the surface of the substrate 16. Althoughunpolarized UV light is used in this embodiment, the non-linearlypolarized UV light, which means that the light is circularly orelliptically polarized, or the light does not polarized can be usedeither. θ₂ is preferably in the range of 0°<θ₂≦60°. After the initialirradiation shown in FIG. 5A, the molecules of alignment layer 15 areset or oriented with a large number of one sided pretilt angledirections, while the magnitude of the pretilt angles formed by each ofthese directions with the surface is substantially the same. After thesecond irradiation shown in FIG. 5B, however, only one of the pretiltangle directions will be selected.

[0033] Thereafter, when linearly polarized UV light 220 irradiates thealignment layer 15 at a perpendicular angle, as shown in FIG. 5B, onlythe pretilt angle direction perpendicular to the polarization directionof this UV light is selected, so that the desired pretilt angledirection can be obtained. Furthermore, the size or magnitude of thepretilt angle produced in the alignment layer 15 varies depending uponthe amount of UV energy absorbed, as noted above with respect to FIG. 4.

[0034]FIGS. 6A and 6B illustrate a third embodiment of the presentinvention. As shown in FIG. 6A, unpolarized UV light 230 irradiatesalignment layer 15 at an angle θ₃ to the normal direction of the surfaceof the substrate, where the θ₃ is in the range of 0°<θ₃≦₆₀°. Thereafter,linearly polarized UV light 240 irradiates alignment layer 15 again atan angle θ₄, as shown in FIG. 6B, where the θ₄ is in the range of0°<θ₄≦60°.

[0035] The resulting orientations of the molecules of alignment layer 15after the steps shown in FIGS. 6A and 6B is similar to that of FIGS. 5Aand 5B, respectively. Namely, after the irradiation shown in FIG. 6A,the molecules of the alignment layer 15 are oriented at a large numberof one sided pretilt angle directions, as in FIG. 5A. Moreover, as inFIG. 5B, only one of these directions is selected after the secondirradiation shown in FIG. 5B. The irradiation direction of the second UVlight forms acute angle with the pretilt angle directions by firstirradiation. In this embodiment, non-linearly polarized UV light,including circularly polarized, elliptically polarized, and unpolarizedUV light, can be used instead of unpolarized UV light. By usingnon-linearly polarized UV light, the method becomes simplified comparedwith the conventional method using linearly polarized UV light.

[0036] A fourth embodiment of the present invention is shown in FIGS. 7Aand 7B. As shown in FIG. 7A, linearly polarized light 250 irradiatesalignment layer 15 at an angle θ₅, 0°<θ₅≦₆₀°, relative to the normal ofthe surfaces of alignment layer 15 to orient the molecules of alignmentlayer 15 at first and second pretilt angle directions, similar to thatshown in FIG. 3A. One of these directions is then selected byirradiating alignment layer 15 at an angle θ₆, 0°<θ₅≦₆₀°, withnon-linearly polarized UV light, specially unpolarized UV light 260,such that all the molecules of alignment layer 15 are oriented at one ofthe first and second pretilt angle directions, as in FIG. 7B. In thisembodiment, the irradiation direction of the second UV light forms acuteangle with one of the first and second pretilt angle directions.

[0037] In aforementioned methods according to the present invention, thesize of the pretilt angle and the two symmetric pretilt angle directionsare determined by an initial exposing an alignment layer including apolysiloxane based material or PVCN-F to UV light. One of these pretiltangles is then selected by a second exposure to UV light. Accordingly,the pretilt angle can be controlled easily.

[0038] While the invention has been described in its preferredembodiments, this should not be construed as limitation on the scope ofthe present invention. Accordingly, the scope of the present inventionshould be determined not by the embodiment illustrated, but by theappended claims and their legal equivalents.

What is claimed is:
 1. A method for controlling a pretilt angledirection for an alignment layer of a liquid crystal cell, comprisingthe steps of: irradiating said alignment layer on a substrate surface afirst time with non-linearly polarized ultraviolet light to orientmolecules of said alignment layer in a large number pretilt angledirection, the pretilt angle directions forming pretilt angles,respectively, relative to a direction parallel to said substratesurface; and irradiating said alignment layer a second time withlinearly polarized ultraviolet light to select one direction of saidpretilt angle directions.
 2. A method for controlling a pretilt angledirection for an alignment layer of a liquid crystal cell according toclaim 1 , wherein a magnitude of said pretilt angles are determined by aduration of said irradiation step first time.
 3. A method forcontrolling a pretilt angle direction for an alignment layer of a liquidcrystal cell according to claim 1 , wherein said pretilt angles are ofequal magnitude substantially.
 4. A method for controlling a pretiltangle direction for an alignment layer of a liquid crystal cellaccording to claim 1 , wherein said alignment layer includes apolysiloxane based material.
 5. A method for controlling a pretilt angledirection for an alignment layer of a liquid crystal cell according toclaim 1 , wherein said alignment layer includespolyvinylfluorocinnamate(PVCN-F).
 6. A method for controlling a pretiltangle direction for an alignment layer of a liquid crystal cellaccording to claim 1 , wherein said step of irradiating said alignmentlayer a first time include a step of irradiating said alignment layer toultraviolet light at an angle relative to a normal direction to saidsubstrate surface, and said step of irradiating said alignment layer asecond time includes a step of irradiating said alignment layer at anangle substantially equal to ninety degree relative to said substratesurface.
 7. A method for controlling a pretilt angle direction for analignment layer of a liquid crystal cell according to claim 6 , whereinsaid angle is in the range of 0 degree to 60 degree.
 8. A method forcontrolling a pretilt angle direction for an alignment layer of a liquidcrystal cell according to claim 1 , wherein said step of irradiatingsaid alignment layer a first time include a step of irradiating saidalignment layer to ultraviolet light at a first angle relative to anormal direction to said substrate surface, and said step of irradiatingsaid alignment layer a second time includes a step of irradiating saidalignment layer at a second angle relative to normal direction to saidsubstrate surface.
 9. A method for controlling a pretilt angle directionfor an alignment layer of a liquid crystal cell according to claim 8 ,wherein a incident direction of said second linearly polarizedultraviolet light forms an acute angle with said pretilt angledirections.
 10. A method for controlling a pretilt angle direction foran alignment layer of a liquid crystal cell according to claim 8 ,wherein said first and second angle are in the range of 0 degree to 60degree, respectively.
 11. A method for controlling a pretilt angledirection for an alignment layer of a liquid crystal cell, comprisingthe steps of: irradiating said alignment layer on a substrate surface afirst time with linearly polarized ultraviolet light to orient moleculesof said alignment layer in first and second pretilt angle directions,said first and second pretilt angle directions forming first and secondpretilt angles, respectively, relative to a direction parallel to saidsubstrate surface; and irradiating said alignment layer a second timewith non-linearly polarized ultraviolet light to select one direction ofsaid first and second pretilt angle directions.
 12. A method forcontrolling a pretilt angle direction for an alignment layer of a liquidcrystal cell according to claim 11 , wherein a magnitude of said firstand second pretilt angles are determined by a duration of saidirradiation step first time.
 13. A method for controlling a pretiltangle direction for an alignment layer of a liquid crystal cellaccording to claim 11 , wherein said first and second pretilt angles areof equal magnitude substantially and of opposite sign relative to saidparallel direction to said substrate surface.
 14. A method forcontrolling a pretilt angle direction for an alignment layer of a liquidcrystal cell according to claim 11 , wherein said alignment layerincludes a polysiloxane based material.
 15. A method for controlling apretilt angle direction for an alignment layer of a liquid crystal cellaccording to claim 11 , wherein said alignment layer includespolyvinylfluorocinnamate(PVCN-F).
 16. A method for controlling a pretiltangle direction for an alignment layer of a liquid crystal cellaccording to claim 11 , wherein said step of irradiating said alignmentlayer a first time include a step of irradiating said alignment layer toultraviolet light at an angle substantially equal to ninety degreerelative to said substrate surface, and said step of irradiating saidalignment layer a second time includes a step of irradiating saidalignment layer at an angle relative to a normal direction to saidsubstrate surface.
 17. A method for controlling a pretilt angledirection for an alignment layer of a liquid crystal cell according toclaim 16 , wherein a incident direction of said second non-linearlypolarized ultraviolet light forms an acute angle with said one of saidfirst and second pretilt angle directions.
 18. A method for controllinga pretilt angle direction for an alignment layer of a liquid crystalcell according to claim 16 , wherein said angle is in the range of 0degree to 60 degree.
 19. A method for controlling a pretilt angledirection for an alignment layer of a liquid crystal cell according toclaim 11 , wherein said step of irradiating said alignment layer a firsttime include a step of irradiating said alignment layer to ultravioletlight at a first angle relative to a normal direction to said substratesurface, and said step of irradiating said alignment layer a second timeincludes a step of irradiating said alignment layer at a second anglerelative to a normal direction to said substrate surface.
 20. A methodfor controlling a pretilt angle direction for an alignment layer of aliquid crystal cell according to claim 19 , wherein a incident directionof said second non-linearly polarized ultraviolet light forms an acuteangle with said one of said first and second pretilt angle directions.21. A method for controlling a pretilt angle direction for an alignmentlayer of a liquid crystal cell according to claim 19 , wherein saidfirst and second angles are in the range of 0 degree to 60 degree,respectively, relative to a normal direction to said substrate surface.